A flower consists of four whorls namely calyx, corolla, androecium and gynoecium, which are arranged on the thalamus. Both, the calyx and the corolla are non-essential whorls of a flower, while the androecium and the gynoecium are essential whorls of a flower. Flowers can be either bisexual or unisexual. A bisexual flower has both androecium and gynoecium, while a unisexual flower has either androecium or gynoecium.
Flowers can also be asymmetrical or symmetrical based on the arrangement of the floral parts around the thalamus. An asymmetrical flower cannot be divided into two similar halves by any vertical plane that passes through its centre while a symmetrical flower can be divided into similar halves. Symmetrical flowers can be sub-divided into two types: actinomorphic and zygomorphic.
An actinomorphic flower can be divided into similar halves in any radial plane that passes through its centre, while a zygomorphic flower can be divided into two similar halves only by a single vertical plane that passes through its axis. Flowers are also classified as trimerous, tetramerous or pentamerous based on the number of sepals, petals and stamens. Some flowers have reduced leaves at the base of the thalamus known as “bracts”. These when present are called bracteate and while absent, called ebracteate.
Based on the position of the gynoecium with respect to the calyx, corolla and androecium on the thalamus, flowers are described as hypogynous, perigynous and epigynous. Hypogynous flower has the ovary at the highest position on the thalamus with respect to the rest of the floral parts. Perigynous flower has the ovary at the centre of the thalamus with the rest of the floral parts on its rim. Epigynous flower has the ovary completely enclosed within the thalamus while all the other floral parts appear above it.
The arrangement of the flowers on the floral axis is called inflorescence, which are of two types. In racemose inflorescence, the flowers appear laterally in acropetal succession, that is, the flowers are arranged laterally from the base to the apex of the main stem. In cymose inflorescence, the flowers appear in basipetal succession, that is, the flowers are arranged at the apex of the main stem and further development takes place by the growth of lateral branches.